1. Field of the Invention
The invention relates to image sensors, and more particularly to a color filter array for improving color imbalance in different regions of an image sensor.
2. Description of the Related Art
Image sensors are necessary components in many optoelectronic devices, including digital cameras, cellular phones, and toys. Conventional image sensors include both charge coupled device (CCD) image sensors and complementary metal oxide semiconductor (CMOS) image sensors.
An image sensor typically includes a plane array of pixel cells, wherein each pixel cell comprises a photogate, photoconductor or a photodiode having a doped region for accumulating a photo-generated charge.
A periodic pattern of dyes of different colors is superimposed over the plane array of pixel cells. This pattern is known as a color filter array (CFA). A plurality of microlenses of a square or a circular shape can be optionally superimposed over the color filter array (CFA) to focus light onto one initial charge accumulation region of each of the pixel cells. Using the microlenses may significantly improve the photosensitivity of the image sensor.
FIG. 1 shows a conventional two-dimensional color filter array (CFA) capable for application in image sensors, including a periodic pattern of dyes of different primary colors such as red (R), green (G) and blue (B) filters. The periodic pattern illustrated in FIG. 1 is a so-called Bayer pattern, including a red (R) filter, a blue (B) filter and a pair of green (G) filters. Moreover, FIG. 2 shows another conventional two-dimensional color filter array capable for application in image sensors, including a periodic pattern of different primary colors such as a cyan (Cy) filter, a magenta (Mg) filter and a pair of yellow (Ye) filters.
In conventional image sensors, undesired issues such as “color shading” may occur in image sensors with the previous mentioned structures and the color shading may be discovered when an image sensor is subjected to a chip probe (CP) test in which an image sensor is examined under a collimated white light. Color shading may occur in the photodiode or photoelectric conversion element of the image sensors when the photodiodes or the photoelectric conversion elements in the image sensor are formed in an irregular pattern rather than a radially symmetrical pattern due to line routing or other device design requirements. An image sensor, suffering from such a color shading issue, may be formed with “image shading” when applied in an optoelectronic device using the image sensor. Thus, causing the optoelectronic device to display abnormal images. Specifically, an uneven or imbalanced image profile presenting, for example, a red-deflected color, a blue-deflected color or other deflected colors at different portions of the image sensor may be formed.
FIG. 3 is a simulated image of an image sensor (not shown) incorporating a CFA arranged in the Bayer pattern illustrated in FIG. 1. The simulated images were obtained by a CP test using a collimated white light, wherein FIG. 3 shows an uneven image profile presenting a substantially yellow-deflected color at a relative upper region 130 of the image sensor and a substantially blue-deflected color at a relative lower region 200 of the image sensor, which are divided from a dotted line 150 formed therebetween. This uneven image profiles illustrated in FIG. 3 is the above described “color separation” and is not desirable in an image sensor because “image shading” may occur in an optoelectronic device employing said image sensor.